听第一段对话，回答小题。

   Bob was cutting a branch（树枝） off a tree in his garden. While he was sawing（锯）, another man passed in the street. He stopped and said, “ Excuse me, but if you continue (继续)to saw（锯） that branch like that, you will fall down with it.” He said this because Bob was sitting on the branch and cutting it at a place between himself and the trunk（树干） of the tree.
   Bob said nothing. He thought, “ This is some foolish（愚笨的） person who has no work to do and goes about telling other people what to do and what not to do.”
   The man continued on his way. Of course, after a few minutes. The branch fell and Bob fell with it.
   “My God!” he cried. “That man knows the future!” and he ran after him to ask how long he was going to live. But the man had gone.

人体健康与食品、药物等关系密切．有些学生从小偏食，不爱吃水果、蔬菜，结果营养缺乏、发育不良，这主要是由于摄取下列哪种物质不足引起的．（　　）

A、在量筒中倒入适量的水，记下水的体积；将小矿石用细线系好后，慢慢地浸没在水中，记下小矿石和水的总体积．

B、把天平放在水平桌面上，把游码移到标尺左端的零刻度线处，调节横梁上的平衡螺母，使横梁平衡．

C、将小矿石放在左盘中，在右盘中增减砝码并移动游码，直至横梁恢复平衡．